Since the advent of the continuous casting of slabs in the steel industry, companies have been trying to combine the hot strip mill with the continuous caster through an inline arrangement so as to maximize production capability and minimize the equipment and capital investment required. The initial efforts in this regard consisted of integrating continuous casters producing slabs on the order of 6 to 10 inches with existing continuous or semicontinuous hot strip mills. These existing hot strip mills included a reheat furnace, a roughing train (or a reversing rougher) and a six or seven stand finishing mill with a capacity of 1.5 to 5 million tons per year. This arrangement is the present day design of large steel company hot mills, and it is unlikely that new hot strip mills of this design would ever be built due to the high capital cost. The quest for low cost integrated caster hot strip mills is not solved by current designs. Further, such prior art integrated mills were extremely inflexible as to product mix and thus current market requirements.
These difficulties gave rise to the development of the so-called thin slab continuous hot strip mill which typically produces 1,000,000 tons of steel per year as standard products. These mills have been integrated with thin slab casters on the order of 2 inches or less. Such integrated thin slab casters are enjoying increased popularity but are not without serious drawbacks of their own. Significant drawbacks include the quality and quantity limitations associated with the thin slab casters. Specifically, the trumpet-type mold necessary to provide the metal for the thin slab can cause high frictional forces and stresses along the surface of the thin wall slab which leads to poor surface quality in the finished product. Further, the 2 inch strip casters are limited to a single tundish life of approximately seven heats because of the limited metal capacity of the mold.
Most importantly, the thin casters, by necessity, have to cast at high speeds to prevent the metal from freezing in the current ladle arrangements. This, in turn, requires the tunnel furnace which is just downstream of the slab caster to be extremely long, often on the order of 500 feet, in order to accommodate the speed of the slab and still be able to provide the heat input to a thin slab (2 inches) which loses heat at a very high rate. Since the slab also leaves the furnace at a high speed, one needs the multistand continuous hot strip mill to accommodate the rapidly moving strip and roll it to sheet and strip thicknesses. However, such a system is still unbalanced at normal widths since the caster has a capacity of about 800,000 tons per year and the continuous mill has a capacity as great as 5 million tons per year. The capital cost of such a system then approaches that of the earlier prior art systems which the system was intended to replace.
In addition, the scale loss as a percentage of slab thickness is substantial for the 2 inch thin cast slab. Because of the extremely long furnace, one must provide a long roller hearth which becomes very maintenance intensive because of the exposed rotating rollers.
It has been suggested that light gauge hot band on the order of 0.040 inch be rolled from these 2 inch slabs. However, in the case of low carbon steels, the thermal decay is too great on a multistand continuous mill making it impossible to achieve the necessary finishing temperatures; and in the case of low alloy high-strength steels, it has been reported that the 2 inch thick slab does not produce the reduction required for high-strength low alloy steel which then causes a coarse microstructure which must then be refined through special temperature treatments which are greater than for the cold charging of the same microalloyed steel grade, "Optimisation of hot rolling schedule for direct charging of thin slabs of Nb-V microalloyed steel", N. Zentara and R. Kaspar, Materials Science and Technology, May 1994.
The typical multistand hot strip mill, likewise, requires a substantive amount of work in a short time which must be provided for by larger horsepower rolling stands which, in some cases, can exceed the energy capabilities of a given area, particularly in the case of emerging countries. Thin slab casters, likewise, are limited as to product width because of the difficulty in using vertical edgers on a 2 inch slab. Further problems associated with the thin strip casters include the problems associated with keeping the various inclusions formed during steel-making away from the surface of the thin slab where such inclusions can lead to surface defects if exposed. Furthermore, existing systems are limited in scale removal because thin slabs lose heat rapidly and are thus adversely affected by the high-pressure water normally used to break up the scale.
In addition, this thin strip process can only operate in a continuous manner, which means that a breakdown anywhere in the process stops the entire line, often causing scrapping of the entire product then being processed.
It is an object of our invention to integrate an intermediate thickness slab caster with a hot reversing mill. It is a further object to adopt a system which balances the rate of the caster to the rate of the rolling mill. It is also an object of our invention to adopt a system using less thermal and electrical energy. It is still a further object to adopt an automated system with small capital investment, reasonable floor space requirements, reasonably powered rolling equipment and low operating costs. It is a further object to provide flexibility in slab sourcing and to economically accommodate increased demand for light gauge wide strip.